Implantable medical devices (IMDs) include devices designed to be implanted into a patient. Some IMDs are designed to provide electrical cardiac therapy to a patient or subject, such as cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities for example. The devices can be used to treat patients or subjects using electrical or other therapy, or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. The devices may be implanted subcutaneously and include electrodes that are able to sense cardiac signals without being in direct contact with the patient's heart. Other examples of IMDs include implantable diagnostic devices such as implantable loop recorders, implantable drug delivery systems, or implantable devices with neural stimulation capability.
IMDs can be single chamber devices (e.g., single chamber ICD) that sense cardiac signals and deliver therapy to a single heart chamber (e.g., the right ventricle) and do not include dedicated atrial sensing capability. Additionally, some diagnostic-only devices do not include dedicated atrial sensing capability. However, patients with these types of devices may develop atrial arrhythmias, such as atrial fibrillation (AF) for example. Knowledge of the amount of time or percentage of time that the patient spends in AF (AF burden) can be useful to physicians for effective titration of drug therapy, especially for heart failure patients who typically have a high incidence of AF.